The present invention relates to control systems and methods for controlling current in an inductive load in the case of a motor or for controlling voltage in the case of a generator, and deals more particularly with a four quadrant unipolar pulse width modulated inverter with which a novel control algorithm is used.
It is well established in the art to use switch mode power conversion techniques for power supplies (DC to DC or AC to DC) and for controlling motor drives. Switch mode power conversion circuits typically use only filter and switch components, that is, inductive and capacitive elements and switch elements which function as either short circuits or open circuits. In the ideal case, there is no power dissipation in the converter. In reality, ideal switch elements do not exist but modern switching devices such as transistors, metal oxide semiconductor field effect transistors (MOSFETS), silicon controlled rectifiers (SCR), gate turnoff (GTO), and other switching devices approximate nearly ideal switching devices provided the number of switch transitions per second is limited. As a consequence, the final circuit design is a result of a set of tradeoffs wherein faster switching frequency results in the ability to use smaller filter components and achieves lower ripple in the controlled variable (current or voltage) but at the penalty of higher switch dissipation requiring larger switching devices which are slower and therefore more limited in frequency.
Although there are numerous and well-known methods for controlling switching devices, pulse width modulation (PWM) techniques are generally preferred because the on-time that the switching device is conducting is changed to match the load demand for power.
Early pulse width modulation controls, generally referred to as voltage mode control, used an averaged value of the output variable which generally was a voltage to determine the pulse width needed.
One of the original current mode control techniques is disclosed in a paper titled "Simple Switching Control Method Changes Power Converter into a Current Source" by Cecil W. Deisch published in 1978 by the IEEE. Deisch discloses a current mode control technique which allows very rapid control response to the current thereby simplifying control stabilization and enabling faster control response to disturbances. Deisch also discloses that there tended to be an instability in a constant frequency current threshold control circuit having duty cycles greater than fifty percent and in these instances proposed that the problem be resolved by utilization of ramp compensation which resulted in a compromise in performance.
U.S. Pat. No. 4,901,366 titled "Circuit Arrangement for the Timed Control of Semiconductor Switches" issued Feb. 13, 1990 to Rottger discloses the use of a full bridge driving an ohmic-inductive load using a PWM technique with unipolar control wherein only one switching element is pulse width modulated to reduce switching losses.
The paper, "Current Mode Control, Five Different Types, Used with the Three Basic Classes of power Converters, Small Signal AC and Large Signal DC Characterization, Stability Requirements, and Implementation of Practical Circuits" by Reidl and Sokal published in 1985 uses current mode controllers for DC to DC converters using hysteretic clocked-on or clocked-off control. Because most DC to DC converters have either a positive output polarity or a negative output polarity but never have to reverse polarities, the reference does not address the problems of applying the technique to a brushless DC motor drive.
The U.S. Pat. No. 4,388,571 titled "Control Circuit for Automobile Electro-magnet Drive Equipment" issued Jun. 14, 1983 to Tada et al. discloses a full bridge which can apply either a positive or a negative polarity voltage to the load wherein one polarity corresponds to the "on" half of the duty cycle of a PWM DC-to-DC controller and the opposite polarity corresponds to the other part of the duty cycle, thus the term bipolar pulse width modulation. Tada discloses the use of a hysteresis band for the controlled variable so that as the variable approached and reached the edge of the band in one direction, the opposite polarity is applied to the bridge to bring the variable in the opposite direction. Tada is useful with DC brush-type motors which have either a positive output for rotation in one direction and a negative output for rotation in the other direction wherein transitions are not a concern. However, when the technique proposed by Tada is applied to all motor drives, it suffers from high switch heat dissipation and exhibits current ripple due to the bipolar drive and also requires synchronization of operation to a single clock at a constant frequency in order to address noise considerations.
The paper entitled "New Constant--Frequency Current--Mode Control for Power Converters, Stable for all Values of Duty Ratio, and Usable in all Four Quadrants" by Anunciada and Silva published in the IEEE Transaction on Industrial Electronics in 1990 discloses the use of a control band around the desired current waveform and the use of dual clock synchronous control over all duty cycle values using a full bridge drive.
Typically, bipolar drive dissipates a great deal of power in non-ideal switching devices. The U.S. Pat. No. 4,477,751 entitled "Motor Brake Device" issued Oct. 16, 1984 to Kanayama discloses the use of a unipolar/bipolar mode PWM control which introduces an alternation during a PWM cycle between one voltage across the load and zero volts across the load instead of bipolar drive alternating between one voltage and its opposite polarity voltage. The result is a lower ripple current in the load, smaller filter components, and lower switch dissipation.
Quadrants may be defined in the voltage and current graph of the load. A positive voltage and positive current is a motor operation which may be either unipolar (one polarity of source voltage applied, alternating with zero voltage applied) or may be bipolar (alternating positive and negative source voltage applied). Reversing the voltage while maintaining the current positive extracts energy from the motor and over short periods reduces the current and over longer periods makes the motor run backwards as a generator. The second and fourth quadrants are thus defined as reversed voltage and reversed current polarity of the load, respectively. The third quadrant represents motoring in the reverse direction and both voltage and current are reversed. Motor controls dealing with a brush-type motor are not concerned with transitions between quadrants since the controls operate in a steady state in one direction or the other direction.
The U.S. Pat. No. 4,562,393 entitled "Modulation Scheme for PWM-type Amplifiers or Motors" issued Dec. 31, 1985 to Loyzim et al. discloses a bipolar PWM control using a bandgap technique, similar to that described in the Anunciada and Silva reference, to drive a brush-type motor. Loyzim does not describe or disclose a unipolar PWM nor does Loyzim address quadrant transition problems.
The U.S. Pat. No. 4,710,868 entitled "Method and Apparatus for Control of Current in a Motor Winding" issued Dec. 1, 1987 to Guzik discloses a unipolar PWM controller for use with a brushless DC motor and Guzik attempts to resolve problems associated with transition between quadrants during which time the current is steeply decreasing and causes loss of control. Guzik discloses a voltage mode control and thus loses the control response advantages of current mode PWM. In order to compensate for the use of voltage mode control, Guzik employs a single turn-on clock rather than the dual clock technique disclosed in the Anunciada and Silva reference. Guzik does not disclose the use of a current band or the possibility for a cycle-by-cycle current control which is an important fault tolerance and reliability consideration. Guzik discloses changing the quadrants with a unipolar PWM control by observing the duty cycle of the PWM to determine if the duty cycle goes to zero or one hundred percent and in which case a loss of control is indicated. At such detection the control changes to select another quadrant to apply the reverse voltage.
It is therefore a general aim of the present invention to provide a control for a full bridge power circuit for driving brushless DC motors which controls current in the inductive load, minimizes switching losses in the bridge switching devices, and avoids loss of control of the current at difficult points in the control waveform such as at zero crossings or when the rate of change is steep.